Among the conventionally-known vehicular braking apparatus are so-called “brake-by-wire” type brake apparatus (hereinafter referred to as “BBW type brake apparatus”) which convert each braking operation by a human driver into an electrical signal to activate an electric fluid pressure production device and then activate brake cylinders with fluid pressure produced by the electric fluid pressure production device. Using such BBW type brake apparatus, it is possible to generate sufficient braking force with a reduced leg power (operating force) that is to be applied to an operation member in the form of a brake pedal.
The BBW type brake apparatus may present the problem that a motor of the electric fluid pressure production device becomes inoperative, for example, due to a failure of a main power supply (battery). To address the problem, Japanese Patent Application Laid-Open Publication No. 11-171006 (hereinafter referred to as “Patent Literature 1”), for example, proposes a brake apparatus which includes not only a main power supply (battery) but also an auxiliary power supply (battery) and which drives the motor using the auxiliary power supply (battery) in the event of a failure of the main power supply. However, the brake apparatus disclosed in Patent Literature 1 requires the auxiliary power supply and members for mounting the auxiliary power supply, which results in an increased number of necessary component parts. In addition, because a separate space for mounting the auxiliary power supply has to be secured, it tends to be time-consuming to consider an appropriate layout.
Therefore, there has been a great demand for a more sophisticated brake apparatus which, in the event of a failure of the main power supply, can operate the brake cylinders without using an auxiliary power supply. One example of such a sophisticated brake apparatus may be constructed in such a manner that, in the event of a failure of the main power supply, the fluid pressure of the master cylinder is increased by a leg power applied to the brake pedal and the brake cylinders are operated by the increased fluid pressure. In order to increase the fluid pressure of the master cylinder, it is preferable to set a lever ratio (i.e., operation amount of the brake pedal/output amount (movement amount) of a push rod connecting to the master cylinder) at a great value. Setting a great lever ratio like this can cause a great operational force to act on the push rod in response to a given operation amount of the brake pedal.
Further, from Japanese Patent No. 3269239 (hereinafter referred to as “Patent Literature 2”) etc. there have been known brake apparatus which are constructed to enhance operability of the brake apparatus by varying a lever ratio in correspondence with an operation amount of the brake pedal (pedal operation amount). With the brake apparatus disclosed in Patent Literature 2, the lever ratio is decreased, i.e. the movement amount of the push rod responsive to the pedal operation amount is increased, in an operation region where the braking force is small, to achieve characteristics that can facilitate operation amount control. In an operation region where the braking force is great, on the other hand, the lever ratio is increased, i.e. the movement amount of the push rod responsive to the pedal operation amount is decreased, to achieve characteristics that can facilitate leg power control. Brake pedal device of the brake apparatus disclosed in Patent Literature 2 is constructed in a manner as shown in FIG. 10 in order to decrease the lever ratio in a region where the braking force is small and increase the lever ratio in a region where the braking force is great.
FIG. 10 is a side view of the brake pedal device disclosed in Patent Literature 2. In the brake pedal device 200, a brake pedal 201 has an upper end portion 201a pivotably mounted to a vehicle body via a first pivot shaft 202, and the brake pedal 201 is connected to a push rod 205 via a connection link 203 and pivot link 204. As a leg power is applied to (i.e., caused to act on) a pedal 207 of the brake pedal 201 as indicated by a white arrow, the brake pedal 201 pivots clockwise about the first pivot shaft 202. The pivot link 204 is pivotably connected to the vehicle body via a second pivot shaft 206.
Behavior of the brake pedal device 200 can be schematically represented in a manner as shown in FIGS. 11A and 11B. Namely, in the schematic representation of FIGS. 11A and 11B, a leg power is applied to the pedal 207 of the brake pedal 201 as indicated by a white arrow, so that the brake pedal 201 pivots clockwise about the first pivot shaft 202. Then, a first connection arm 208 of the brake pedal 201 presses the connection link 203 as indicated by a clockwise arrow via a first connection pin 209. Also, the connection link 203 presses a first arm 204a of the pivot link 204 via a second connection pin 211, so that the pivot link 204 pivots counterclockwise about the second pivot shaft 206. Then, a second arm 204b of the pivot link 204 moves a push rod 214 (corresponding to the push rod 205 of FIG. 10) via a third connection pin 213 as indicated by a leftward arrow, so that a piston 216 of a master cylinder moves as indicated by the leftward arrow.
FIG. 12 is a graph showing relationship between a pedal operation amount and lever ratio in the conventionally-known brake pedal device, where the vertical axis represents the lever ratio while the horizontal axis represents the pedal operation amount. In a pedal operation amount region 0-s1, the lever ratio increases as the pedal operation amount increases, as indicated by a curve g1. In a pedal operation amount region s1-s2, the lever ratio decreases as the pedal operation amount increases. Further, in a pedal operation amount region s2-s3, the lever ratio increases as the pedal operation amount increases. According to the curve g1, the brake pedal device 200 is set to be used in a pedal operation amount region s4-s5. Thus, it is possible to decrease the lever ratio in an initial brake pedal operation region e1 where the braking force is small and increase the lever ratio in a following brake pedal operation region e2 where the braking force is greater.
In this case, in order to increase the fluid pressure of the master cylinder by the leg power applied to the brake pedal 201 so that the brake cylinders can be operated by the applied leg power alone, the brake pedal 201 has to be moved up to the greatest possible operation amount s5. However, the lever ratio r1 when the brake pedal 201 is at the greatest possible operation amount s5 is lower than the maximum lever ratio r2. Thus, even with the brake pedal device 200 disclosed in Patent Literature 2, it would be difficult to operate the brake cylinders by increasing the fluid pressure of the master cylinder. Consequently, in order to increase the fluid pressure of the master cylinder up to operating pressures of the brake cylinders, the leg power to be applied by a human operator or driver has to be increased, which would impose an increased load on the human driver.